Several pulsed NMR tools have been designed for borehole deployment via wireline and are described in U.S. Pat. Nos.:
4,350,955, issued September, 1982 to Jackson et al.; PA0 4,629,986, issued December, 1986 to Clow et al.; PA0 4,710,713, issued December, 1987 to Striktan; PA0 4,717,876, issued January, 1988 to Masi et al.; PA0 4,717,877, issued January, 1988 to Taicher et al.; PA0 4,717,878, issued January, 1988 to Taicher et al.; PA0 4,714,881, issued December, 1987 to Givens; PA0 5,023,551, issued June, 1991 to Kleinberg et al.; PA0 5,055,787, issued October, 1991 to Kleinberg et al.; and PA0 5,055,788, issued October, 1991 to Kleinberg et al. All of these patents describe NMR tools which employ permanent magnets to polarize hydrogen nuclei, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, or permeability of a formation, for example. The tools described in U.S. Pat. Nos. 4,717,877; 5,055,787; and 5,055,788 have been successfully tested in boreholes.
The instrument concepts described in the related U.S. Pat. Nos. 4,710,713 and 4,717,877 have a rotationally invariant region of resonance, which is a thin cylindrical shell coaxial with the borehole. The electromagnetic fields have a simple azimuthal dependence: both the static and RF fields at any point in the vicinity of the instrument rotate at the same rate as the instrument rotates. The rotation does not disturb the NMR measurement because the nuclear magnetic moment adiabatically tracks the magnetic field. The radial thickness of the resonance region of these instruments is on the order of 1 mm.
The instrument concepts described in the related U.S. Pat. Nos. 4,350,955, 4,629,986, and 4,717,876 have rotationally invariant static and RF fields and a relatively large resonant region (at least 10 mm thick). The resonant region is thicker because the magnitude of the magnetic field is stationary at the resonant region. These patents, however, do not describe rotating the instruments.
Several logging while drilling (LWD) and measuring while drilling (MWD) tools have been designed for formation evaluation while drilling and drill string characterization while drilling, respectively. Logging or measuring instruments are placed in drill collars up to 100 ft behind the drill bit. An MWD tool is described in U.S. Pat. Nos. 3,777,560 to Guignard and 4,479,564 to Tanguy. An LWD tool is described in U.S. Pat. Nos. 4,899,112 to Clark et al. and 4,949,045 to Clark et al. Typically, these tools use electromagnetic techniques in evaluating resistivity of a formation while drilling.
None of the NMR tools have been used in a drilling string, nor have the LWD or MWD tools used pulsed NMR techniques.